Study On Anchoring Mechanism Of Under-reamed Pressure Ground Anchor

For low cost, simple construction technique, short construction period, groundanchor is widely used in field of architecture, water conservancy, electric power, road, slopereinforcement and pit support. However, tension anchor is commonly used, existingproblems of low bearing capacity, long anchoring length, poor corrosion resistance anddurablity. Under-reamed and multi-under-reamed pressure anchors are new type anchordeveloped in recent years, which can effectively improve anchoring effeciency, and forcingstate and corrosion problems. For lack of recoginition of anhoring mechanism and systemticresearch, they were designed in method of tension anchor designing. By means of elasticmechanics method, comparative study of field test data, numerical simulation, the anchoringmechanisim and design method of under-reamed pressure anchor is studied, thenmulti-under-reamed pressure anchor is then studied. The main contents of the works are asfollows:(1) According to forcing analysis of under-reamed pressure anchor, two type of failuremodes were assumed, which are soil yield in front of reamming segments and pull-out of thewhole anchoring body. On basis of elasticity theory, the shear stress and anxial forcedistribution is derived in shape of negative exponential distribution. Furthermore, the forcingcharacterisitics of multi-under-reamed anchor was studied by the principle of superposition.Further analysis was made to study the influencing factors on anchoring effect, such asfactors of design parameters(anchoring length and reaming borehole diameter), mechanicalparameters (cohension, internal friction angle and stiffness coefficient), bearing depth andbearing body number,etc.(2) By making use of the insititution-made reaming driller, field test at two site wasundertaken to study the anchoring mechanism of under-reamed and multi-under-reamedpressure anchor. The pull-out tests were done and load-displacement curves were achieved,and strain of reamed anchoring body was monitored during the test.(3) Based on results of field tests, numerical analysis was done with FLAC3D. Thebearing capacity and stress distribution was gotten, and inluence of anchoring body layoutwas analyzed. Comparitive analysis was conducted on computing results of anchors withdifferent type, also results of numerical results with those of field tests, it was showned thatunder-reamed pressure anchor could get higher bearing capacity and lower stressconcentration. (4) With Kelvin’s fundamental solution，the additional stress and displacement field ofsingle under-reamed pressure anchor was anlayzed, and results of group anchors werederived by superimposing of single anchor. The resonable spacing of anchors was thenachieved and group effect was characterized by group effect coefficient and interactioneffect coefficent.(5) The design method of under-reamed pressure anchor was studied, the equal peakstress method and continuous stress method was proposed to design the reaming segmentlength and effective anchoring length.The paper forwards particularly grate to Shenzhen Land Bureau for the supportingand sponsorship of the subject on "New technologies study on control of geological disastersin Shenzhen area ".